The invention concerns a method to recover a portion of oil bypassed by a steam override zone during a steamflood by recompleting a production well into the interval underlying the oil producing zone.
It is well recognized that primary hydrocarbon recovery techniques recover only a portion of the petroleum in the formation. Thus, numerous secondary and tertiary recovery techniques have been suggested and employed to increase the recovery of hydrocarbons from the formations holding them in place. Thermal recovery techniques have proven to be effective in increasing the amount of oil recovered from the ground, particularly for heavy oils. Steamflooding has been the most successful thermal recovery technique yet employed in commercial practice. However, steamflooding may still leave up to 60% to 70% of the original hydrocarbons in place, depending on the formation and the quality of oil.
When an oil reservoir is subjected to steam injection, steam tends to move up in the formation, and condensate and oil tend to move down due to the density difference between the fluids. Gradually, a steam override condition develops, in which the injected steam sweeps the upper portion of the oil zone, but leaves the lower portion untouched. Injected steam will tend to follow the path of least resistance from an injection well to a production well. Thus, areas of high permeability will receive more and more of the injected steam which further raises the permeability of such areas. This phenomenon exists to an even larger degree with low injection rates and thick formations. The steam override problem worsens at greater radial distances from the injection well because steam flux decreases with an increase in steam zone radius.
Although residual oil saturation in the steam swept region can be as low as 10%, the average residual oil saturation in the formation remains much higher due to poor vertical conformance. For these reasons, increasing vertical conformance in steamfloods by reducing the amount of oil bypassed by a steam override zone has long been a concern of the oil industry.